TN (Twisted Nematic) type and STN (Super-Twisted Nematic) type liquid crystal display devices using a nematic liquid crystal material are well-known. However, such liquid crystal display devices cause screen disturbance or lower contrast when driven at high speed because their electrooptical response rate is of the order of a millisecond. Furthermore such liquid crystal display devices have only a limited display capacity, and are not therefore suited to the display of a motion picture. In addition, the viewing angle of TN type and STN type liquid crystal display devices is too narrow to be used for a large-scale screen. Thus, considerable interest is being shown in liquid crystal display devices using ferroelectric or antiferroelectric liquid crystal material for such applications.
In 1975, R. B. Mayer et al. synthesised DOBAMBC (2-methylbutyl p-[p(decyloxybenzylidene)-amino]-cinnamate) on the assumption, based on the symmetry property theory, that, if optically active molecules have a dipole moment perpendicular to the major axis of the optically active molecule, the molecules exhibit a ferroelectric property in the chiral smectic C phase (SmC* phase), and succeeded in demonstrating this ferroelectric property in the liquid crystal material for the first time (see J. Phys. (Parts) 36 (1975) L69, R. B. Mayer, L. Liebert, L, Strzelecki and P. Keller).
FIG. 8 (a) shows a model of a smectic layer structure within a liquid crystal material and the molecular alignment in the ferroelectric SmC* phase. Although the position of the centre of gravity of the molecules within the layer is arbitrary, as shown schematically in the figure by cones 101, the major axis (director 102) of each liquid crystal molecule is tilted at a certain angle .theta. with respect to the normal z to the smectic layer 103. The tilt direction of the director 102 is shifted slightly between each layer, so that the liquid crystal material exhibits a helical structure. The helical pitch is approximately 1 .mu.m which is substantially greater than an interlayer space of 1 nm or so.
Clark and Lagerwall discovered that the helical structure is lost as the cell thickness approximates to 1 .mu.m (substantially as thick as the helical pitch), and instead, as shown in FIG. 8 (b), the molecules 104 in each layer are switched to one of two stable states in response to an applied voltage. Based on this discovery, Clark and Lagerwall devised a surface stabilised ferroelectric liquid crystal (SSFLC) cell, examples of which are disclosed in Japanese Laid-open Patent Application No. 107216/1981 (Tokukaisho No. 56-107216) and U.S. Pat. No. 4,367,924.
In FIG. 8 (b), an electric field E which extends perpendicularly, from the rear to the front of the drawing, is applied to the molecules 104. In addition, the electric dipole moment of each molecule 104 is aligned along the direction of the electric field E, as illustrated inside each molecule 104 in FIG. 8 (b).
As shown in FIG. 9, the molecules 104 in the SSFLC are in one of two stable states A and B depending on the orientation of the applied electric field. In state A, the electric field -E, which is oriented from the front to the rear of the drawing, is applied to the molecules 104, whereas, in state B, the electric field +E is oriented in the opposite direction.
As shown diagrammatically in FIG. 9, the SSFLC is placed between a polariser having a polarisation axis indicated by the arrow P.sub.1 and a polariser (analyser) having a polarisation axis indicated by the arrow P.sub.2 in such a manner that the major axis of the molecule is aligned along the axis P.sub.1 in state B, for example. The SSFLC cell therefore transmits light in state A to produce a light state and blocks the light in state B to produce a dark state. Thus the SSFLC cell acts as a black-and-white display in response to switching of the direction of the applied electric field.
In the SSFLC, a drive torque is generated by interaction of the spontaneous polarisation and the applied electric field. Therefore, in contrast to a conventional nematic liquid crystal device using dielectric anisotropy, the response rate to the electric field is of the order of a microsecond. Moreover, once the SSFLC has been switched to one of the two stable states, the SSFLC can remain in the same stable state even after the electric field has been removed, so that the SSFLC has a memory function. Thus, a voltage does not have to be applied to the cell all of the time.
As explained, the required display contents can be written into the SSFLC cell by scanning line-by-line using its characteristics of fast response and memory function. When combined with passive matrix driving circuitry, such a SSFLC cell structure enables a display of large capacity to be realised, which can be used in a flat television set to be hung on a wall, for example.
Thus a SSFLC device possesses the advantage over other liquid crystal devices, such as the twisted nematic liquid crystal device, that it is a bistable device which can be switched between two states by switching pulses of alternate polarity and which will remain in one state in the absence of a switching pulse until a switching pulse of appropriate polarity is applied to switch it to the opposite state. By contrast, in operation of a twisted nematic liquid crystal device, a drive signal must be applied continuously to maintain the device in one of its states. SSFLC devices are of particular interest in multiplexed applications as the level to which such devices can be multiplexed is not restricted by the need to re-address particular pixels within a very short time frame.
Strictly speaking, a ferroelectric liquid crystal display device can display only two transmission levels, that is light and dark, because of the bistability of the liquid crystal molecules in the SmC* phase. However, in practice, the FLC display device can realise greyscale to some extent by using time modulation (temporal dither), spatial modulation (spatial dither), or the fast modulation of the applied electric field. However the application of such techniques can cause the structure of the driving system and/or the panel production process to become complicated, thus undesirably increasing manufacturing costs.
In order to change the state of a SSFLC it is necessary to apply a voltage signal of a particular magnitude and polarity across the device for a finite duration. However, in certain applications, it is possible to arrange for partial switching of the device by reduction of the voltage and/or duration of the signal applied across the device so that only a part of the liquid crystal material in the relevant area of the device changes state. Where the device is a display device and the two fully-switched states correspond to black and white states, such partial switching can be used to provide an analogue greyscale. However the control of the greyscale is rendered difficult by the difficulty in controlling domain formation during partial switching of the device.
A number of proposals have previously been made for controlling domain formation in a ferroelectric liquid crystal device. For example a technique is disclosed in JP 03048819 (Matsushita) and JP 04127124 (Asahi Glass) for fabricating arrays of microstructures within the device to provide nucleation points. However this technique requires extra cell fabrication steps which add to manufacturing difficulty and cost. Furthermore E. Matsui and A. Yasuda, FLC 95 Abstracts (1995) 97-99 and EP 0595219A (Sony) discloses a technique in which small balls are distributed in the liquid crystal material to act as nucleation points. However it is difficult to obtain good uniformity of these balls within the material.
Japanese Laid-open Patent Application No. 194635/1994 (Tokukaihei No. 6-194635) (Reference No. 1) and EP 0586014A (Philips) discloses a technique for forming a structure in which non-reactive chiral liquid crystal molecules are captured an anisotropic 3-D network structure made of a polymeric substance. According to this technique, the network structure stabilises microscopic domains having opposite polarisation directions, so that a greyscale can be maintained even after the applied electric field has been removed. However, because of the relatively high concentration of polymer which remains within the liquid crystal material and the 3-D network structure, the viscosity of the liquid crystal material is increased and this tends to lead to slower switching.
Furthermore, Japanese Laid-open Patent Application No. 248489/1995 (Tokukaihei No. 7-248489) (Reference No. 2) discloses a technique for forming within a liquid crystal material a microscopic 3-D network synthetic resin extending along the direction in which the alignment treatment is applied. This technique involves irradiating a mixture of the liquid crystal material and a synthetic resin material with ultraviolet radiation at a temperature at which the mixture remains in the nematic phase. In this manner a greyscale can be realised by forming a plurality of domains which are separated out of the 3-D network synthetic resin material and each of which has a respective threshold voltage.
However, in the technique of Reference No. 1, the switching domains are not regular and do not remain the same size when a voltage is applied. Further, since the liquid crystal molecules are constrained by the 3-D network structure, the response rate to the applied electric field is slowed down considerably, as the liquid crystal molecules are in the monostable state in some areas. Also, if the ratio of chiral materials added to the host liquid crystal material is increased, the liquid crystal material is not aligned in a satisfactory manner, thereby causing the display quality to deteriorate.
Similarly, in the technique of Reference No. 2, the regularity and size control of the domains are not satisfactory. This gives rise to the problems that the liquid crystal molecules are constrained by the 3-D network synthetic resin and the contrast is lowered. Furthermore, since the domain size is not sufficiently small as compared with the pixel size (0.3 mm square), it is almost impossible to realise a greyscale display in practice.